In reprocessing spent nuclear fuels and/or blanket materials, phosphoric acid esters (mixed with hydrocarbons) are frequently used as solvent for extracting actinide elements. When such solvent is used for a prolonged period of time, radiolysis or chemical reactions give rise to undersized decomposition products. Although some of these products can be removed by cleaning steps, no complete separation is possible. Because of the pronounced complexing properties of decomposition products there is an increased, disturbing extraction of fission products, such as zirconium-95, from the aqueous phase into the organic phase loaded with actinides. However, an increase in fission product concentration of the organic phase not only reduces the extraction efficiency for the actinides and the separation efficiencies both of the actinides from the fission products and the actinides from each other and the degree of purity of the individual actinides but also enhances the radiolytic processes in the organic phase and, in addition, aggravates the phase separation by generating turbidities and colloids in the interface between the organic and aqueous phases.
For full utilization the solvent should be recycled. However, in that case a cumulation of unfavorable influences cannot be avoided, and the usefulness of a solvent batch is limited despite the decontamination step following every application. The rate at which a batch becomes unusable for further application is a function of such factors as the burnup level of the nuclear fuels and blanket materials, respectively, their concentration in the aqueous phase, the number of recycles of a batch (number of application steps), etc.
Solvent mixtures which have become useless in this way, or must be termed useless, represent strongly radioactively contaminated organic wastes and are separated first into their phosphoric acid ester and hydrocarbon constituents by means of the addition of phosphoric acid, the ester, e.g. tributyl phosphate (TBP), combining with phosphoric acid into an adduct insoluble in hydrocarbons, which adduct is further decomposed after separation from the hydrocarbons. The hydrocarbons can be removed by burning.
Various methods of TBP removal have so far been employed, but either they are unfeasible because of their environmental impact or they require relatively large expense in terms of time, facilities, cost, etc. It was suggested, for instance, to burn liquid organic wastes containing phosphoric acid esters. However, this generates highly corrosive gases carrying with them radioactive material and phosphorpentoxide aerosols. The usual type of filters for gases containing radioactive materials are plugged up within a very short time, corrode and thus become ineffective. The combustion or flue gases, therefore, must first be scrubbed and the phosphorpentoxide must be neutralized before there can be a final filtering step.
Moreover, it was suggested to discharge all solutions containing TBP into the ground in arid areas with a low population density. However, only some of the radioactive materials will be retained in the soil components, while most of them will pass through the layers of soil together with the organic liquid and may reach ground water.
Distillation processes can be used only for the more or less effective separation of TBP from hydrocarbons, but this neither removes the TBP nor prepares it in any way for non-polluting storage.
In addition, disadvantages connected with the methods outlined above give rise to some hazards, such as radioactive materials getting into the biocycle, organic liquids entering ground water, generation of easily flammable gases, and explosions in distillation plants.
Another technique which has been suggested in the prior art is the incorporation of TBP into polyethylene, as disclosed in U.S. Pat. No. 3,463,738 to Fitzgerald et al. Products produced by mixing polyethylene, with the addition of heat with either a mixture of tributyl phosphate and alkane hydrocarbon or with tributyl phosphate which had previously been separated from the alkane hydrocarbon, are solid gels which, during storage in containers for a period of time exhibit undesirable shrinkage as the result of a discharge of liquid. The solid polyethylene-containing bodies no longer contact the container walls. This shrinkage results in an environmental danger because the containers may break, for example, by mechanical force when the containers are stacked, or due to corrosion, and this breakage would enable the radioactive liquid to escape. Since the possibility of this discharge liquid entering the biocycle cannot be dependably excluded, the process for solidifying and removing radioactive organic waste liquids with the aid of polyethylene is unsuitable in view of the safety and environmental protection problems involved.
In an article by Burns, Solidification of Low- and Intermediate-Level Wastes, ATOMIC ENERGY REV., 9(3) pages 583 to 584, Sept., 1971, it is stated that limited experiments were carried out which show that it is possible to introduce chemical sludges, which contained about 50% water, into plastic wastes comprised of a mixture of polyethylene and polyvinyl chloride. In discussing the results obtained with the waste plastic mixture of polyethylene and polyvinyl chloride Burns states that the tests were experimental in nature, that the activity in wastes was kept to tracer levels, and that it would be advisable to carry out further experiments before coming to definite conclusions concerning the use of such waste plastic mixtures with aqueous chemical sludges. Burns does not disclose that such waste plastic mixtures can be used for solidification of organic phosphoric acid ester wastes.